The invention pertains to novel methods for drawing fibers from liquid melts, including those composed of materials whose liquid state viscosity at the melting temperature is normally too low to accommodate fiber drawing operations. The invention also pertains to methods of drawing fibers from a melt while preventing recrystallization of the melt. Further, the invention pertains to methods of controlling heat transfer at melt surfaces so that a portion of the melt is undercooled to a temperature below the equilibrium melting temperature, and fibers can be drawn from the undercooled portion of the melt. The invention also relates to the production of novel fibers including but not limited to fibers of glass and crystalline materials, glass fibers formed using the methods of the invention, and fibers of high tensile strength compared to fibers of the same composition which are currently commercially available.
Melt-Drawn Fibers
The drawing of fibers from liquid melts is well known in the art as an inexpensive method of fiber synthesis. This process is possible if the liquid viscosity is (1) sufficiently high so that tensile forces overcome the surface tension forces of the liquid during the fiber drawing process, and (2) sufficiently low so that the tensile forces induce liquid flow into a thin fiber rather than bulk flow of the liquid.
Drawing is widely used to make fibers from materials that form viscous melts, such as but not limited to oxide compositions that contain a high concentration of silicon dioxide and polymeric materials. The silica glass fibers and polymeric fibers have considerable economic utility as, for example, thermal insulation material, components in composite materials, use in textiles, and for many other applications. Fiber-drawing is used to make glass fibers for applications such as, for example, fiber lasers and fiber-based optical devices. These glass fibers typically contain small concentrations of optically-active dopant elements which are added to the host glass, such as neodymium (Nd) in Nd-glass lasers and erbium (Er) in Er-glass lasers. The magnitude and the uniformity of the dopant concentration in the fibers are limited, however, by the solubility and diffusivity of the dopant oxides in a preform of the host glass material during the doping operation that occurs prior to fiber drawing from the preform.
In some cases, mixtures of several pure materials yield melts from which fibers can be drawn. For example, fluoride glass fibers are made by melt drawing from mixtures of several metal fluorides that exhibit a low melting temperature, thereby forming relatively viscous melts. Fluoride glass fibers provide optical transmission outside the bandwidth of silica-based fibers, and are of interest for applications such as fiber-laser and infrared waveguide applications. However, considerable difficulties generally attend the manufacture and use of most fluoride fibers, such as brittleness, moisture sensitivity and toxicity. In addition, many mixtures of fluoride materials or pure liquid fluorides have insufficient equilibrium melting temperature viscosities for fiber drawing operations. Improved alternatives to these prior art fluoride fibers are needed. As used herein, the xe2x80x9cequilibrium melting temperaturexe2x80x9d, xe2x80x9cor the melting temperaturexe2x80x9d of a melt is the temperature at which at equilibrium, all of the melt components of a system are substantially liquid.
It is the temperature, at the prevailing ambient pressure and for a system whose chemical composition is equal to the chemical composition of the melt of interest, for above which no crystalline phases occur in a system at equilibrium.
Oxide glass fibers are often made from mixtures of silicon dioxide, boron oxide, sodium oxide, and other additives, which mixtures melt at temperatures much lower than the melting temperature of pure silica and result in melts sufficiently viscous for the drawing of fibers.
Fibers known as chalcogenide glass fibers can also be drawn from mixtures of elements such as germanium, arsenic, antimony, selenium, tellurium, and others which form viscous, low melting temperature liquids. Chalcogenide glass fibers have application in the transmission of infrared radiation. However, many chalcogenide materials have equilibrium melting temperature viscosities which are too low to accommodate fiber drawing operations.
Prior art methods also limit the concentrations of additives that may be achieved in glass fibers. For example, optically-active additives for fiber laser and fiber laser amplifier applications are generally introduced into pre-formed host glass fibers by heating the host fiber in the presence of the additive materials, which are generally applied to the outside of the drawn fiber (such as, for instance, by spraying). With this method, the additive concentration in the fiber is limited to the equilibrium solubility of the additive material at the heating temperature. Fibers made by this process, however, suffer from the drawback that the heating temperature, and therefore the additive concentrations, are limited by crystallization of the host fiber.
Drawn or extruded fibers are also used as precursor fibers in chemical or physical processes that change the fiber material into a different chemical form or physical state. For example, sol-gels formed from metal-organic chemicals can be drawn or extruded into fibers of an amorphous material that is subsequently heated to decompose the organic fraction of the fiber and produce polycrystalline oxide fibers. Silicon-containing organic polymer materials can be drawn into fibers and subsequently decomposed to form silicon carbide fibers. Organic polymer fibers made from polyacrylonitrile (PAN) are heated and decomposed at high temperatures to obtain carbon fibers. Pitch compositions obtained from hydrocarbon or coal tars can be drawn into fibers and subsequently decomposed at high temperatures to obtain carbon fibers. Polycrystalline oxide fibers of various materials such as zirconia-silica materials, alumina-boria materials, alumina-silica materials, and yttria-alumina materials have been made from precursor fibers that are formed by drawing or extrusion processes.
Prior art methods of fiber manufacture include drawing fibers from undercooled melts. As used herein, an xe2x80x9cundercooledxe2x80x9d temperature refers to a temperature which is below the melting temperature of the combined melt components. For instance, the melting temperature of aluminum oxide is 2050 degrees C. An undercooled melt of aluminum oxide would be a liquid melt held at a temperature below this. Fibers containing one or more of calcium oxide, aluminum oxide, silicon dioxide, magnesium oxide, and/or barium oxide have been drawn by conventional means utilizing partial undercooling by melting the starting materials in a platinum crucible or by melting the end of a rod of the starting material, removing the source of heating allowing the melt to cool, contacting the undercooled surface of the liquid with a glass rod, and manually drawing a fiber from the undercooled surface of the liquid by withdrawing the glass rod and attached fiber. Alternatively, the center section of a rod of the starting material is melted, the heating source is removed, and a fiber is formed when the two ends of the rod are manually drawn apart. Reportedly, fibers could be drawn using these methods from melts that contained a maximum of 46.2% aluminum oxide by weight, which is equivalent to a maximum of 35.3 molar % of aluminum oxide, Al2O3. However, it is reported that melts which contain more than 50% aluminum oxide by weight have much lower viscosities, and fibers of these higher-alumina composition materials could not be drawn from melts using these prior art techniques, even when the drawing portions of the melts were undercooled.
The methods described above are limited in part by the fact that the melt is in contact with a solid rod of the same material or with a platinum container. As the melt cools, crystals propagate from the solid/liquid interface to consume the liquid portion and thus limit the duration of time the melt remains liquid for fiber drawing. A second limitation of using traditional methods is that fibers cannot be drawn from the melt for binary Al2O3xe2x80x94CaO compositions, but only for compositions with at least 3.5% by weight of added silicon oxide or with 17.8 weight % of added barium and magnesium oxides. Finally, fibers cannot be drawn using these methods from melts containing a binary mixture of 50 molar % calcium oxide and 50 molar % aluminum oxide, i.e., the chemical composition CaAl2O4, because the viscosity of such melts is too low to permit the drawing of fibers by the prior art methods described above.
In general, the melts formed at the equilibrium melting temperature for the vast majority of pure materials are of a viscosity that is too low to support fiber drawing operations. The number ofxe2x80x9csuperheatedxe2x80x9d (melt temperature above the equilibrium melting temperature) molten materials having viscosities sufficiently large for fiber drawing is also limited.
In a typical fiber drawing operation, the melt must be of sufficiently high viscosity to enable fibers to be drawn from the melt. In general, the nominal viscosity required for fiber drawing from the liquid is from approximately 1,000 to 1,000,000 poise. Viscosity values for most liquids, however, are much lower than those required for fiber drawing. For example, typical viscosities are 0.01 poise for water, 1-100 poise for molten oxides and slags that do not contain silicon dioxide, 0.01 to 0.1 poise for molten salts, 0.01-1 poise for metals and alloys.
As discussed above, undercooling is one method sometimes used to increase the viscosity of a melt. At least slight undercooling can be achieved in most liquids. However, where a liquid melt is cooled in contact with a solid container, the container may induce the nucleation of solid material from the liquid melt, resulting in the solidification of the entire mass of the melt. In addition, contact with a container can introduce impurities into the melt, as a result of dissolution of the container material into the melt. Such contamination of melts by contact with a container is a problem in, for example, the fabrication of fluoride glass and chalcogenide glass fibers used for transmission of infrared light, in the fabrication of high purity fibers, and in the fabrication of high melting temperature materials intended for service in high temperature structural applications. In addition, contact with a container may also induce crystallization of the melt.
As a result of the low viscosity of many liquids and because crystallization due to contact with containers limits undercooling, there is traditionally a very limited range of materials from which fibers can be made by drawing from a liquid melt placed within a container.
In addition to undercooling, a second method of increasing the viscosity of a melt is by adding viscosity enhancers such as SiO2 to the melt. Some liquids containing silicates have a high viscosity and an ability to be undercooled without recrystallization, i.e., to be cooled below the melting temperature without undergoing spontaneous recrystallization. The ability to undercool such materials facilitates the drawing of fibers, since the viscosity of the liquid melt may be increased by decreasing the temperature of the melt to a value at which the optimum drawing viscosity is obtained.
The utility of silicate fibers is limited, however. For example, the presence of silicon oxide in the silicate fibers leads to increased absorption of infrared radiation relative to fluoride materials, chalcogenide materials and to other oxide materials such as aluminum oxide or yttrium-aluminum oxide, the silicates do not conduct electricity, the silicates limit laser action of laser active dopants, and the silicates may be chemically reactive towards matrix materials in high temperature composite materials. These and other limitations inherent in silicates have led to the development of many alternative fiber forming methods, some of which are described below.
Containerless Systems for Melt-Drawn Fibers
Another means of increasing the range or the purity of materials from which drawn fibers may be produced is to use a containerless system. Containerless systems typically involve the levitation of a liquid drop, or small amounts of solid material, by generating forces on the levitated specimen which compensate for the force of gravity. The forces used to levitate the specimens may be produced by aerodynamic, electromagnetic, acoustic, electrostatic and any other means or combination of means of levitating specimens.
One such containerless system involves separating the liquid from the surface of a container used to shape, position, or mold the liquid. The separation occurs by means of a gaseous film formed by gas permeating through the wall of the container. This method is generally applicable to relatively large masses of liquid. This method has been used to levitate molten mixtures of metal fluorides and to draw fibers from the fluoride melts, whose viscosity is sufficiently high at temperatures above the equilibrium melting temperature to permit the drawing of fluoride fibers. In this way, fluoride fibers of a greater purity are obtained than would be possible if the melts were held in a container, which would dissolve when exposed to the corrosive fluoride liquid and contaminate the melt.
Another containerless system for drawing fibers involves use of a levitation furnace apparatus which levitates a material, heats and melts the material, initiates drawing of the material and results in the drawing of a fiber from the levitated liquid, which is cooled as it is drawn from the levitated liquid.
Yet another system involves an apparatus and method for drawing optical glass fibers from the molten tip of a glass precursor in a self-supported, containerless environment. The method employs melting of the glass precursor rod in a temperature gradient furnace such that a liquid drop forms at the end of the rod and a fiber is drawn from the melt. The process, however, requires a microgravity environment, and is not useful for melts which exhibit low surface tensions.
Containerless conditions may, also be obtained by the use of an aero-acoustic levitator (xe2x80x9cAALxe2x80x9d). An AAL levitates the-liquid drops from which fibers may be drawn by the use of aerodynamic forces from a gas jet, and the levitated sample is stabilized by application of acoustic forces from a three-axis acoustic positioning system. This method is generally used to levitate specimens with diameters in the range of approximately 0.25 cm to approximately 0.35 cm, although larger and smaller specimens may also be levitated by using this method.
Devices used to levitate 0.25 to 0.40 cm diameter specimens include, for example, conical nozzle levitation (xe2x80x9cCNLxe2x80x9d) devices in which a levitation gas flow passes through a plenum chamber, through a nozzle and over the specimen, levitating the specimen. The levitated specimen is then heated and melted with a laser beam.
Other Fiber-Forming Methods
In addition to drawing fibers, several other processes have been developed to form fibers from low viscosity melts. One such process is the xe2x80x9cedge-defined film-fed growth technique,xe2x80x9d in which a single crystal fiber is formed by crystallization of liquid. The diameter of the fiber formed is determined by interfacial forces and by the diameter of a small orifice in the container that supplies the liquid.
Another process is the xe2x80x9cpedestal growth technique.xe2x80x9d In this process, the end of a small rod of starting material is melted by the application of focused laser beam heating, and a smaller diameter single crystal filament is drawn from the melt. Careful control of the linear growth rate of the filament and of the heat transfer conditions at the point of crystallization allows fibers to be formed by crystallization of the melt. An advantage of this process relative to the edge-defined film-fed growth technique is that higher purity materials can be made because no container is required.
The materials to which the edge-defined film-fed growth technique and the pedestal growth technique have been applied include aluminum oxide, yttrium aluminum garnet, ceramic superconductor materials, and others. Single crystal filaments are obtained, with chemical compositions of the fibers equal to the chemical composition that is in equilibrium with the melt. The resulting fibers are typically of 50-100 micrometers (xe2x80x9cxcexcxe2x80x9d) or larger in diameter and the linear growth rates at which the fibers are formed are typically less than 1 cm/second. In contrast, however, non-crystalline fibers drawn from viscous melts can be made with diameters less than 1 micrometer or larger, and the fibers are formed at very high rates of several hundred cm/second.
Yet another process for forming fibers and filaments from melts of low viscosity employs extrusion of the liquid as a free stream into an atmosphere which forms a stabilizing film on the stream, after which solidification occurs within the stabilizing film. For example, low viscosity melts may be extruded into a hydrocarbon atmosphere where a solid carbon film is formed on the liquid stream. However, the fibers formed by this method are contaminated by the carbon sheath, which must be removed in subsequent processes, and may also be contaminated by interaction with the container material.
In addition, the technique of-chemical vapor deposition (xe2x80x9cCVDxe2x80x9d) has been used to form small diameter filaments of one material deposited on a core fiber of another material. Examples of commercially available CVD filaments are boron filaments deposited on thin tungsten substrate fibers and silicon carbide filaments deposited on carbon fiber substrates. These filaments are typically of 100 micrometers in diameter or larger. Laser CVD also can be performed in which fibers are formed at the focus of a laser beam, inside a CVD reactor. Laser CVD has been used to make pure fibers of boron, silicon carbide, silicon nitride, silicon, germanium, and carbon, with diameters of approximately 10 micrometers or larger. However, Laser CVD is a slow process in which the fibers are formed at linear growth rates of 0.1 cm/second or slower.
It thus would be advantageous to have a method for drawing fibers from materials which exhibit insufficient viscosities to permit fiber drawing at their equilibrium melting temperature. It would be additionally advantageous to have drawn fibers from materials which in the prior art are thought to be incompatible with a conventional fiber-drawing process. Additionally, it would be useful to have a means of minimizing recrystallization of molten materials during fiber drawing. Still further, it would also be advantageous to produce novel fibers from melts containing dissolved additives in larger concentrations than previously available in drawn fibers produced by conventional xe2x80x9cdopingxe2x80x9d operations, including fibers with additives whose concentrations exceed the equilibrium concentration of the additives in the fibers at the fiber crystallization temperature. Lastly, it would be advantageous to have a means of producing crystalline fibers with controlled chemical compositions.
The present invention provides a method for drawing fibers from liquids which exhibit insufficient viscosities for fiber drawing at and above the equilibrium melting temperatures. Further, the present invention provides novel draw fibers from materials which were thought to be incompatible with a fiber-drawing process. The present invention also provides a means of minimizing recrystallization of a bulk melt during fiber drawing. Still further, the present invention provides novel drawn fibers with high concentrations of additives as compared to conventionally drawn prior art fibers, including fibers with additives whose concentrations exceed the equilibrium concentration of the additives in the fibers at the fiber crystallization temperature. In addition, the present invention provides fibers of a higher tensile strength than prior art fibers of the same composition. Lastly, the present invention provides a means for producing crystalline fibers with controlled chemical compositions.
The present invention achieves these objectives by heating the desired materials until completely melted thus forming a melt, undercooling the melt until the proper viscosity is reached and maintaining the melt at the undercooled temperature, initiating fiber drawing by inserting a xe2x80x9cstingerxe2x80x9d into the melt and rapidly withdrawing the stinger from the melt, then finally drawing the fibers at the desired speed so that fibers of the desired composition and diameter are formed. If desired, crystalline fibers can be formed by heating the drawn fibers until crystallization occurs. The present invention also provides for drawing fibers from an undercooled melt held under either containerless conditions or within a container. As used herein, the term xe2x80x9cundercooled meltxe2x80x9d refers to a melt or a portion of a melt that is held at an undercooled temperature (i.e., at a temperature below the melting temperature) during fiber drawing operations.